The human eye functions to provide vision by refracting light through a clear outer portion called the cornea, and refracting the light by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become aberrated, vision deteriorates because of the loss of retinal image quality. This loss of optical quality in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. During this procedure, a portion of the anterior capsule is removed and a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquefies or emulsifies the nucleus and cortex of the lens so that the lens may be aspirated out of the eye. The diseased nucleus and cortex of the lens, once removed, is replaced by an artificial lens in the remaining capsule (in-the-bag). In order to treat corneal astigmatism, the IOL may be toric. The power selection for the IOL can also take into account the effect of incisions in the corneal shape (astigmatism) through which the IOL is injected, as described in greater detail in U.S. Pat. No. 7,476,248, which is incorporated herein by reference.
An alternative treatment for corneal astigmatism is the use of limbal relaxing incisions (LRIs), which are typically opposed arcuate incisions in the cornea that reshape the cornea to correct astigmatism. While the term “limbal relaxing incision” or “LRI” is used in this specification, because the incisions are typically made at the limbus and therefore conventionally described this way, the term should be understood to include any corneal relaxing incision, referring generally to any incision that does not penetrate the cornea and that is positioned to adjust the astigmatism of the cornea. Based on statistical samples of surgical outcomes, two commonly used nomograms, the Donnenfeld nomogram and the Nickamin nomogram, have been developed to guide surgeons in performing LRIs. The Donnenfeld nomogram takes into account age (relatively to average age for cataract patients), incision location and pattern, and whether the astigmatism is with the rule or against the rule. The Nickamin nomogram considers age and degree of astigmatism more granularly than the Donnenfield nomogram. These nomograms can be further customized based on the surgeon's actual surgical outcomes.